Compound droughts and hot extremes: Characteristics, drivers, changes, and impacts

Compound droughts and hot events or extremes (CDHEs) may lead to larger repercussions than do individual extremes. Due to the disastrous impacts and increased risk of these events under global warming, increased attention has been paid to them from both research and operational communities. This review provides a synthesis of the literature on characteristics, drivers, changes (detection, attribution, and projection), and impacts of CDHEs. Different characteristics of these events (e.g., frequency, duration, and spatial extent) are first introduced based on drought and hot indicators at different time scales. We then summarize multiple physical mechanisms of CDHEs, including the atmospheric circulation (and modes of variability) and land-atmosphere feedbacks across different regions. Evidence from observations shows an overall increase in CDHEs in the past few decades at regional and global scales, which mainly results from an increase in hot extremes and is likely attributable to anthropogenic influences. Future projections indicate an increase in CDHEs over most global land areas. Quantitative assessments of the impact of CDHEs on different sectors (e.g., water resources, crop yield, vegetation) highlight their amplified impacts compared with individual droughts or hot extremes. Several challenges in the data availability, characterization, physical mechanism, simulation, and impacts of CDHEs and opportunities to address these challenges are then discussed. This study can be useful for better understanding, modeling and risk analysis of compound extremes under global warming.